Field of the Invention
This invention relates to shaping glass sheets by press bending when the glass sheets are suspended from tongs. It is well known that freely suspended tongs tend to hang in a vertical orientation. Glass sheets are gripped at their upper edges by a plurality of tongs when the glass is to be heated to its deformation temperature for shaping or heated to a temperature sufficient for tempering followed by rapid cooling. The use of tongs minimizes the mass of material that tends to absorb heat in a furnace provided for heating the glass sheets to their desired elevated temperature. Tongs have been used to provide a more efficient furnace operation than other means for conveying glass sheets that are more massive and that absorb a greater proportion of the furnace heat. Also, tongs provide a minimum disruption of the cooling operation that is used to develop a temper in the heated glass sheets.
Traditionally, tongs have been suspended from carriages which rode on overhead conveyors that carried the tongs and glass sheets gripped by the tongs through a tunnel-like furnace, a shaping station and a cooling area. Initially, the tongs were suspended from rigid points of support provided by the carriage.
Unfortunately, when glass sheets are bent, the portions gripped by the tongs are displaced from the vertical plane in which the flat glass sheet is initially supported. Some of the upper edge portions of a glass sheet gripped by the tongs move in one direction relative to the thickness of the glass, while other portions of the upper edge of the glass sheet move in the opposite direction during a glass sheet bending operation. When the tongs gripping the glass are freely hung from rigid points of suspension, the tongs tend to revert to a vertical hanging position, thereby causing the glass sheet to develop dimples or kinks in the tong gripping portions. The severity of each of these dimples or kinks is a function of the horizontal distance that each gripped portion of the glass sheet moves from the position it occupies prior to bending.
When glass sheets are bent about a sharp axis of bending, particularly in their central portion, the length of the glass sheet shortens considerably and the tongs which grip the glass sheet on opposite sides of the sharply bent portion are required to follow the gripped points of the glass a considerable horizontal distance toward the central axis of sharp bending. Unless the tong suspension points move distances equal to the movement of the tong gripping points as the glass sheet is shaped, the tongs become suspended obliquely and tend to move the glass gripping points apart after the molds, which shape the glass sheet, separate and release the obliquely suspended tongs to tend to return to vertically hung positions, which tend to distort the glass sheet.
When glass sheets are bent to non-symmetrical bends about a sharply bent vertical axis of bending, the tong gripping points move different horizontal distances toward the vertical axis. It is necessary that the tong suspension points move distances that are approximately equal to the movements of the glass gripping points. Otherwise, the tongs assume oblique hanging positions which tend to cause kinking and they tend to force the glass sheet to be distorted after the shaping molds are retracted. In addition, in asymmetrical bends, the outermost portion of the convex mold preferably engages and remains engaged with a reference point on the glass sheet from its time of initial contact until both molds engage the sheet completely. Asymmetrically hung tongs may cause the glass sheet to slide relative to the convex mold during the interval between initial engagement and final, complete engagement. This sliding may cause the sharply bent region to be displaced from its desired location.
In the past, many solutions have been attempted to minimize kinking of the glass due to tong engagement. Many of these involve moving the point of tong suspension in response to glass sheet shaping. For one reason or another, as will be explained in a more detailed analysis of prior patents, the prior solutions left something to be desired. A need still existed for a simple device that enables the tong suspension points to follow the movement of the tong gripping points of the glass sheet involving a minimum of moving parts and a sufficiently rigid tong support capable of withstanding the wide temperature range to which the tongs and the tong support structure are subjected during thermal tempering of glass sheets that would avoid glass kinks.